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Multi-layer dielectric coatings can be made more reflective than over-coated aluminum but only at one angle. Reflectivity changes both in percentage and in wavelength according to the angle of reflectance.
Accordingly, multi-layer dielectric coating is OK for a star diagonal or a secondary mirror in a newtonian, where the angle of reflection is a particular angle with a small variation from side to side.
On a primary mirror, though, especially one with a fast focal ratio, a ray reflects from the mirror at widely differing angles depending on which section of the mirror is being reflected from.
I suppose it would be possible to vary the coating materials in circular segments but the process would require masking many times and it would be very complicated.
So the best coatings for a primary, reflectivity-wise, remain aluminum with 3-5 layer overcoating (call "enhanced" coatings). Or overcoated silver, if recoated at more frequent intervals than aluminum.
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